A well known building block for digital and analog electronic circuits is the operational amplifier or differential amplifier. A basic operational amplifier configuration may be used to make comparisons between the voltage level of two input signals. An operational amplifier may have multiple uses in different circuit configurations such as amplifying an input signal to a desired level using different circuit components, performing mathematical functions such as addition or multiplication, and signal modulation.
However, present operational amplifiers have several problems. Present operational amplifiers are analog circuits having components which add electrical noise to the basic sensor signal and are sensitive to signal saturation. Each of the two input sensor signals on a conventional operational amplifier must first be preamplified before they can be differentially processed. This preamplification is commonly done using transimpedance amplifiers that "sample" the sensor current with a summing junction node. The output of the transimpedance amplifier produces a voltage which forces a current (via a feedback resistor) back into the node to offset the current produced by the sensor. The resultant output voltage is therefore a "transform" of the sensor current. Use of the transimpedance amplifier functions well except the sensor current is easily contaminated by amplifier noise.
Signal and amplifier noise may be reduced by the use of optical switch sensors which are impervious to stray electronic noise. Previously, optosensors included a single photodiode, phototransistor, photodarlington, and the like are two-state, current-driven devices that have an "on" or "off" current state. For applications such as optocouplers and optoisolators, these devices responded to an "on" or "off" pre-couple signal with a corresponding "on" or "off" post-couple current signal. The inherent speed of such devices was limited by the rate at which they could switch their currents "on" and "off," the limiting factor often being the passive return-to-ground period. Also for an "on" current state to be recognized, the current had to be at a significantly greater amplitude than background noise. However, the higher the signal current that was needed to generate this recognition, the longer time required by the switch device to generate that current level, and an even longer period was required before the switch device would return to the ground level. These characteristics of previous optoelectronic switches resulted in relatively slow switching speeds of usually less than 1 MHZ for a standard photodiode, and even slower speeds for more complicated devices such as phototransistors.
An improved faster optoelectronic switch, termed an opsistor, has been proposed in our co-pending application, Ser. No. 08/755,729, now U.S. Pat. No. 5,837,995, to the same inventors. However, although such optoelectronic switches can be designed to respond with faster switch frequencies by using special circuitry, the additional components of such circuitry increase the complexity and cost of such devices. Further, the transmitter and receiving elements of fast optoelectronic switches have to be in close proximity, usually in a single package, for efficient function and to minimize extraneous light interference.
Ideally, the sensor current should be sensed by FET high impedance probes that do not contaminate the signal purity. The sensor current itself must flow in a circuit free of contamination currents. To achieve this, the sensor current has to be confined in an "electrical loop" where it can flow with high purity. This property, however, has no utility if the sensor current magnitude cannot be "read." Therefore, this ideal circuit must also produce a detectable voltage change proportional to its sensor current. In addition, the operating point DC bias across this sensor must be at a fixed voltage (usually 0 volts) to achieve sensor linearity. The transimpedance amplifier satisfies some of these conditions but suffers from the fact the feedback resistor must be in contact with the actual sensor input, thus injecting amplifier noise that is also amplified along with the sensor signal. These requirements of a closed loop current path and a pure input signal cannot be simultaneously met with teachings from current art.
Thus, there is a need for an operational amplifier which may minimize input signal noise. There is a further need for a simple optical operational amplifier which may use presently known circuit components. There is also a need for a simple optical operational amplifier which allows an adjustment of output gain. There is a need for an optical operational amplifier which includes passive components to shape an input waveform.